This application is based on and hereby claims priority to Japanese Application No. 046467 filed on Feb. 23, 2000 in Japan, the contents of which are hereby incorporated by reference.
Wavelength division multiplexed (WDM) amplifiers amplify optical signals that are composites of multiple wavelength optical signals. WDM optical communications systems relay multi-wavelength composite optical signals through multiple optical amplifiers.
The band over which losses are low in optical fiber transmission circuits (less than approximately 0.3 dB/km) is the band from 1450 nm to 1650 nm. As shown in FIG. 1, a variety of optical fiber amplification devices have been developed for this transmission band.
At present, with the popularity of cellular telephones and the rapid increase in internet use, the demand for telecommunications capacity is expanding explosively. There are global intense research and development efforts for technologies that can increase the information transmission capacity on a single fiber.
Optical wavelength division multiplexing (WDM) technology that uses the broadband characteristics of optical fiber amplifiers having silica erbium-doped fibers (EDF) is critical. The conventional wavelength band is known as both xe2x80x9cthe 1550 mn bandxe2x80x9d (1530 to 1560 nm) or xe2x80x9cthe C bandxe2x80x9d (conventional-wavelength band).
In addition, EDF optical amplifier equipment for a 1580 nm band (1570 to 1600 nm) xe2x80x9cthe L bandxe2x80x9d (longer-wavelength band) has been developed. The competition has become intense in developing a commercial optical fiber telecommunications system that is able to transmit an ultra large capacity (perhaps 1.6 terabit/s) of information by modulating each multiplexed wavelength at 10 Gb/s with about 80 waves in each of the bands for a total composite of 160 waves.
Because there is a capacity of approximately eight THz when C band and L band are combined, when 10 Gb/s transmission signal channels are established with the 2.5 GHz spacing, the overall transmission capacity of 1.6 terabit can be expanded further up to 3.2 Tb/s   (            10      ⁢              xe2x80x83            ⁢              Gb        /        s            xc3x97              8,000            ⁢              xe2x80x83            ⁢      GHz              25      ⁢              xe2x80x83            ⁢      GHz        )
On the other hand, there is demand for even greater carrying capacity, and so optical fiber amplification devices that have new optical amplification bands, in addition to the current C band and L band, are required.
In FIG. 1, even though GS-TDFA (gain-shifted thulium-doped fluoride-based fiber amplifiers) are being developed for amplification in the S band region from 1490 nm to 1530 nm, GS-TDFA devices have a gain in the region between 1475 and 1510 nm, and thus it may be difficult for them to succeed in the portion of S band extending from 1510 to 1530 nm.
In addition, the 1610 to 1650 nm band is limited to specialty fibers that are either thulium or terbium-doped fluoride-based fibers.
In the optical amplifier devices described above, the optical amplification medium amplifies light through excited emission, which occurs from population inversion of energy levels. There is also Raman fiber amplification, which uses the non-linear effects of fibers. Because Raman fiber amplification makes use of the non-linear effects of fibers, it can produce a gain in any given wavelength band by selecting the wavelength of the stimulating light source. However, there are problems in that the gain per unit length is small, so the optical amplification fibers must placed every several kilometers to every several dozen kilometers within the transmission line.
An optical amplifier according to one aspect of the invention includes an optical amplification medium, an excitation source to stimulate the amplification medium to output at least one wavelength gain peak, and a gain equalizer to equalize the output of the amplification medium such that gain is produced at wavelengths other than the wavelength gain peak. The gain equalizer may attenuate gain at the peak wavelength. The gain equalizer may equalize the output of the amplification medium such that gain is produced at wavelengths less than the wavelength gain peak.
A variable attenuator and automatic level circuitry may be provided such that the automatic level control circuitry monitors at least one of the input of the optical amplifier and the output of the optical amplifier and maintains the output level of the optical amplifier at a substantially constant level.
The optical amplification medium may be formed from a plurality of amplification medium structures which together produce at least one wavelength gain peak when stimulated by the excitation source. The amplification medium structures may be semiconductor optical amplifiers. Also, the gain equalizer may be formed of a plurality of gain equalizer segments, which together produce gain at wavelengths other than the wavelength gain peak. The gain equalizer segments may be substantially transparent to the pumping wavelength of the excitation source and may be positioned with amplification medium structures positioned therebetween.
The excitation light source may stimulate the optical amplification medium to achieve a population inversion rate having a positive gain throughout an optical signal wavelength band. The wavelength gain peak may be outside of the optical signal wavelength band. The gain equalizer may attenuate the wavelength gain peak.
The optical amplification medium has an input and an output. A feedback hoop to the excitation source may monitor the input and output of the amplification medium and maintain a substantially constant gain within the amplification medium over time. Specifically, an automatic gain control circuit may be connected to monitors at the input and output to control the excitation source so as to maintain a constant gain within the amplification medium over time.
The optical amplification medium may be located within a resonator. The optical amplification medium has an input and an output, and the resonator may include a pair of mirrors that reflect a selected wavelength and optical couplers provided at the input and the output of the amplification medium to divert a portion of the light emitted from the optical amplification medium to the mirrors. The optical couplers may be 9:1 couplers. The mirrors may be fiber grating mirrors. The gain equalizer may be substantially transparent to the selected wavelength. The selected wavelength reflected by the mirrors may be within a signal band used for optical signals, as long as no optical signal to be amplified is transmitted at the selected wavelength.
The optical amplification medium may have a cladding, a doped core provided interior to the cladding, and gratings provided within the highly doped core.
Another aspect of the invention may have an amplification medium formed of at least one erbium doped fiber, an excitation light source to produce a population inversion ratio of about 0.7 to about 1.0 within the amplification medium, and a gain equalizer to obtain substantially identical wavelength characteristics for a wavelength band of from about 1490 nm to about 1530 nm. The excitation light source may supply pumping light to the amplification medium at a pumping wavelength, such that the gain equalizer is substantially transparent to the pumping wavelength. For a wavelength band of from about 1450 nm to about 1490 nm, a population inversion ratio of about 0.8 to about 1.0 may be used. For a wavelength band of from about 1610 nm to about 1650 nm, a population inversion ratio of about 0.3 to about 1.0 may be used.
According to an optical amplification method, a population inversion ratio is selected to achieve positive gain throughout an optical signal wavelength band. The amplification medium is excited to the selected population inversion ratio to produce a wavelength gain peak at a wavelength outside of the optical signal wavelength band. Gain is equalized to achieve substantially uniform gain over the optical signal wavelength band. Amplification in wavelength bands outside of the optical signal wavelength band is attenuated. The optical signal wavelength band may be at wavelengths less than the wavelength of the wavelength gain peak for the amplification medium.
According to yet another aspect of the invention, a WDM splitter separates first and second different optical signal wavelength bands (for example, the C-band and the L-band). An optical amplification device for the first wavelength optical signal band includes a first amplification medium, an excitation light source to produce a first population inversion ratio within the first amplification medium, and a gain equalizer to obtain substantially uniform gain over the first optical signal wavelength band. An optical amplification device for the second wavelength band includes a second amplification medium and an excitation light source to produce a second population inversion ratio within the second amplification medium. The first and second population inversion ratios are different. A WDM coupler recombines the first and second optical wavelength bands after amplification.
The first population inversion ratio may be larger than the second population inversion ratio, for example, assuming that the first optical wavelength band is the C-band and the second is the L-band. The first and second optical amplification mediums may each be formed of a rare earth element doped optical fiber. In this case, the length of the rare earth element doped optical fiber for the first amplification medium may be greater than that for the second amplification medium.
The first amplification medium may have a wavelength gain peak, outside of the first optical signal wavelength band. The WDM splitter may separate first, second and third different optical signal wavelength bands. In this case, the optical amplifier includes an optical amplification device for the third wavelength band.